1. Field of the Invention
This invention relates to solar thermal energy converters.
2. Description of the Prior Art
Numerous devices have been proposed for utilizing solar energy as a source of heat, particularly for home heating needs. Commercialization of such devices, however, has been inhibited because of the typically high initial capital costs of the solar heating systems presently available.
One approach to reducing costs of solar heating systems involves increasing collector efficiency so as to reduce the total number of solar collectors required. High efficiency collectors, however, require special energy absorbing coatings on the absorber plate and/or plastic heat traps, for example, which minimize convection heat losses. One such selective absorber coating, for example, is disclosed in U.S. Pat. No. 3,958,554. A particularly useful device for preventing loss by thermal convection is disclosed in U.S. Pat. No. 4,019,496. These features, on the one hand, increase the thermal efficiency of solar collectors employing them but on the other hand necessitate that such solar thermal collectors be constructed of expensive materials which will sustain not only the high operating temperatures but also the significantly high temperatures that result as the heat load requirements of the collector system decreases. In other words, collectors, and particularly high efficiency collectors, must be designed to withstand temperatures and pressures under no flow or stagnant conditions, and such a design requires expensive materials of construction, and/or means for modulating the temperature within the collector. Another approach to reducing the costs of solar thermal systems is to fabricate solar collectors from relatively inexpensive materials such as plastics. In these instances, protecting the collector against thermal damage is also of vital importance.
One technique proposed for protecting solar collectors from damage that may result from excessively high temperatures existing within the collector for lengthy periods of time requires the venting of the collector using ambient air and thermally actuated valves. U.S. Pat. No. 4,043,317 and U.S. Pat. No. 4,046,134 are exemplary of such type of air venting systems.
Another technique used for protecting solar collectors from thermal damage is disclosed in U.S. Pat. No. 4,102,325. This patented system provides the heat exchange loop for rejecting excess heat to the atmosphere.
Yet another technique for protecting the solar collector from the hazards of excessively high overtemperatures involves shading or otherwise blocking the incident solar radiation from falling on the absorber within the collector at predetermined temperature conditions. U.S. Pat. No. 4,112,918 is illustrative of this technique. Indeed, in this regard, it is worth noting that polarizing windows can be employed to reduce, i.e. to block, the amount of light transmitted through a window and hence incident on the interior of a structure having such a window. An example of such polarizing window is disclosed in U.S. Pat. No. 4,123,141.
Notwithstanding the foregoing technologies, there remains a need for a solar heat collector which is not only simple in construction but lower in cost, thereby overcoming some of the drawbacks of solar to thermal energy converters of the prior art.